Treatment of certain cardiovascular diseases such as aortic aneurysms or chronic heart failure often includes positioning devices within the aorta or the coronary sinus through catheterization. Such procedures require precise placement of the implanted devices within the target lumens, and can result in severe complications if such implantations are inaccurate. Accordingly, having a clear map of the aorta and/or coronary sinus minimizes the risks involved in these procedures.
An aortic aneurysm is a ballooning of the wall of an artery resulting from the weakening of the artery due to disease, heredity, aging or other conditions. When an area of the aortic wall weakens, the pressure of the blood flowing through the weakened area causes the vessel wall to balloon out, forming a blood-filled aneurysm sack. Although most aneurysms are initially small, aneurysms tend to enlarge over time. Left untreated, the aneurysm will frequently rupture, resulting in loss of blood through the rupture.
Aortic aneurysms are the most common form of arterial aneurysm and are life threatening due to the massive internal bleeding that results from rupture. In the past 30 years, the occurrence of abdominal aortic aneurysms (“AAA”), in particular, has increased threefold. According to the Society of Vascular Surgeons, ruptured aneurysms account for more than 15,000 American deaths each year, making the AAA the thirteenth leading cause of death in the United States.
The aorta is the main artery that carries blood from the heart to the rest of the body. The aorta arises from the left ventricle of the heart, extending upward and bending over behind the heart, and thereafter extending downward through the thorax and abdomen. The abdominal aorta supplies the two side vessels to the kidneys, the renal arteries. Below the level of the renal arteries, the abdominal aorta continues to about the level of the fourth lumbar vertebrae where it divides into the iliac arteries. The iliac arteries supply blood to the lower extremities and perineal region. Accordingly, the aorta is a major arterial component of the circulatory system and maintaining its general condition is critical to the overall health of the patient.
AAA is the most common type of aortic aneurysm. Specifically, AAA is an aneurysm that occurs in the portion of the abdominal aorta that is particularly susceptible to weakening between the renal arteries and the iliac arteries. While, in other areas of the aorta the indication that intervention is necessary is when the aneurysm reaches about 5 cm in diameter, an aortic aneurysm larger than about 4 cm in diameter in this section of the aorta is ominous. Left untreated, the AAA may rupture, resulting in rapid and usually fatal hemorrhaging.
Although the mortality rate for an aortic aneurysm is extremely high (about 75-80%), there is also considerable mortality and morbidity associated with surgical intervention to repair an aortic aneurysm. This intervention typically involves going through the abdominal wall to the location of the aneurysm in order to bypass or replace the diseased section of the aorta. A prosthetic device, typically a synthetic tube, is often used for this purpose. The graft serves to exclude the aneurysm from the circulatory system, thus relieving the pressure and stress on the weakened section of the aorta at the aneurysm.
Repair of an aortic aneurysm by surgical means is a major operative procedure. In addition, substantial morbidity accompanies the procedure, resulting in a protracted recovery period. Finally, the procedure entails a substantial risk of mortality. While surgical intervention may be required in spite of these risks, certain patients may not be able to tolerate the stress of intra-abdominal surgery. It is desirable to reduce the mortality and morbidity associated with intra-abdominal surgical intervention.
In recent years, methods have been developed to attempt to treat an aortic aneurysm without the attendant risks of surgical intervention. One such minimally invasive alternative is endovascular aneurysm repair (“EVAR”). EVAR treatment involves the placement of an endovascular stent in the aneurismal area of the aorta through a percutaneous technique. Specifically, in most circumstances, the endovascular stent is inserted into a blood vessel (artery or vein), usually through an entry site located in the upper leg or neck. Under fluoroscopy, the stent is navigated through the blood vessels until it reaches the aorta where it is positioned over the aneurysm.
While EVAR has been reported to have a lower mortality rate as compared to open surgical repair, to effectuate a successful delivery of EVAR, it is necessary to have a clear map of the aorta such that the stent can be properly positioned. Although the aneurysmic regions of the aorta may be quite diseased and atherosclerotic, for the procedure to be successful a healthy portion of the aorta must be present to serve as a landing zone for the stent. For example, securing the stent to a diseased region of the aorta will result in a faulty seal that will not adequately reroute the blood flow away from the aneurysmic region, thereby resulting in a reoccurrence of the condition. As accurate placement of the stent is critical, visualization of the aortic structure has been an obstacle for proper navigation during delivery of the stent. Currently, clinicians perform magnetic resonance imaging prior to delivering the stent in order to supply an axial profile of the aorta. However, magnetic resonance imaging is expensive and time consuming to obtain, and the results exhibit limited spatial resolution.
Chronic heart failure (“CHF”) is another cardiovascular disease, the treatment of which often includes catheterization. CHF is a disease condition in which the heart fails to function efficiently as a pump and cannot provide sufficient blood flow and/or pressure to satisfy the normal circulatory needs of a patient. A patient with acute CHF often experiences sudden shortness of breath, fainting, and irregular heart beats that require frequent emergency room treatments. In its chronic form, CHF leads to repeated hospital stays, a deterioration in quality of life, and significant costs to the healthcare system.
In about 30% of CHF patients, the disease process compromises the myocardium's ability to contract, which thereby alters the conduction pathways through the heart. A healthy heart has specialized conduction pathways in both the atria and the ventricles that enable the rapid conduction of excitation (i.e. depolarization) throughout the myocardium. Normally, the sinoatrial node (“SA node”) initiates each heart-beat cycle by depolarizing so as to generate an action potential. This action potential propagates relatively quickly through the atria, which react by contracting, and then relatively slowly through the atrio-ventricular node (“AV node”). From the AV node, activation propagates rapidly through the His-Purkinje system to the ventricles, which also react by contracting. This natural propagation synchronizes the contractions of the muscle fibers of each chamber and synchronizes the contraction of each atrium or ventricle with the contralateral atrium or ventricle.
When a patient exhibits damage to the electrical system of the heart, as is often seen in patients with CHF, severe issues may arise. Disruption of the conductance pathways through the heart can cause a delay in the beginning of right or left ventricular systole and thereby induce asynchronous atrial and ventricular activation. Electrocardiographically, this dysynchrony is manifested as a long QRS interval. Alterations in ventricular contractility frequently compromise the ability of the failing heart to eject blood and may consequently increase the severity of the regurgitant flow through the mitral valve. In patients exhibiting these severe symptoms, the intraventricular conduction delays lead to clinical instability associated with a greatly increased risk of death.
Since 2001, approximately 271,000 heart failure patients in the United States have received cardiac resynchronization therapy (“CRT”) to treat moderate to severe heart failure (“HF”). Conventional CRT methods employ a pacemaker to pace both ventricles of the heart such that the heart can resynchronize. CRT devices have three leads; the first positioned in the atrium, the second positioned in the right ventricle, and the third inserted through the coronary sinus to pace the left ventricle.
Due to the required placement of the third lead, the implantation and maintenance of a CRT device are associated with a greater risk than the implantation and maintenance of a standard pacemaker device. Primarily, it is a difficult procedure to advance the pacing lead into the coronary sinus and cardiac veins and, thus, implantation fails in approximately 8% of patients. Further, in approximately 6% of patients, implantation is compromised by dissection or perforation of the coronary sinus or cardiac vein. Severe complications are associated with the inaccurate implantation of a pacing lead, including complete heart block, hemopericardium, and cardiac arrest (which, together, occurred in about 1.2% of patients).
As accurate placement of the leads is critical, the ability to visualize a map or profile of the coronary sinus is important to ensure proper navigation. Conventionally, clinicians use cardiac angiography to visualize the lumen of the blood vessels and the heart chambers. Cardiac angiography typically involves using a combination of injections of radiocontrast agent or dye and x-ray fluoroscopy to visualize the position and size of blood vessels within the heart. This process, however, is not particularly accurate and does not provide a detailed profile of the coronary sinus.
Thus, there is a need for an efficient, accurate, easy to use, and reasonably priced technique for determining the longitudinal profile of the aorta and the coronary sinus.